SRAM Patent Shows Mechanical T-Type Transmission With Smart Setup Feature

SRAM has published a patent application for a mechanical direct-mount derailleur. Currently, SRAM's Transmission derailleurs are entirely wireless and electronically controlled, featuring a direct-mount design that eliminates the need for a separate hanger, providing a stiffer, more precise connection to the frame for improved shifting and durability.

This new patent strengthens the possibility that a mechanical version of the direct-mount system could be on the horizon.

What stands out in the patent is SRAM’s approach to simplifying chain tension and B-gap adjustment in the mechanical design. Unlike Transmission’s direct-mount system, which lacks a B-tension adjuster screw, the derailleur’s rotational position must be set manually before tightening the securing bolt. According to the patent, this process involves fitting the correct chain length, shifting into the designated "setup cog"—which could be the 21-tooth or 24-tooth depending on the bike—then rotating the derailleur backward until the chain is taut before securing it in place.

In a mechanical version, the return spring in the derailleur’s parallelogram would make it difficult—or even impossible—to keep the derailleur in the setup cog while rotating and torquing it into place. Much of the patent focuses on potential solutions SRAM could use to address this challenge.

In its simplest implementation, a hex key would be inserted vertically through two holes in the derailleur’s parallelogram, locking the mechanism in place while setting chain tension. Different-sized hex keys could correspond to specific gears, adapting to various bike types. The patent refers to this as the "reference sprocket," typically the 6th or 7th sprocket on the cassette, which determines the ideal derailleur position.

For full-suspension bikes, which require different B-gap settings due to chain growth, this method eliminates the need to measure B-gap at sag. For instance, a 5 mm hex key could be used to set the derailleur position for a hardtail, while a 4 mm hex key might be needed for a full-suspension bike, positioning the derailleur on a different sprocket to account for suspension movement.

The patent also explores alternative methods for temporarily securing the derailleur while setting chain tension, including designs that eliminate the need for a separate tool. These could involve built-in locking mechanisms or other integrated features that simplify the setup process.

The patent also says that the derailleur cage could be modular and replaceable, as it is with the electronic version:

"This configuration of the chain guiding arrangement together with the pretensioning device to form a sub-assembly enables a modular construction of the rear derailleur. Another advantage involves, in the event of a defect in the chain guiding arrangement or in the pretensioning device, the entire rear derailleur not having to be replaced, but instead only the sub-assembly being able to be replaced."

Cearly SRAM has been refining the mechanics of a potential mechanical Transmission derailleur. While it doesn’t guarantee a production model is imminent, it strongly suggests that such a version could be in the works and could be released this spring with the arrival of new bike models from some of the early adopters of the T-Type system such as Santacruz and Specialized.

It will be interesting to see if this new mechanical T-Type derailleur becomes an immediate option for lower-tier models, with the primary choice being between electronic and mechanical versions. While the benefits of the direct-mount design are clear, the advantages of an electronic derailleur are less so.